1. Field of the Invention
The present invention relates to an image fixing apparatus mounted on an image forming apparatus, such as a copy machine and a printer, that adopts an electrophotographic method or an electrostatic recording method and forms an image on a recording material.
2. Description of the Related Art
In an image fixing apparatus for heating and fixing toner onto a recording material, when the recording material on which fixing processing is performed is small in size, it is known that a phenomenon (a rise of a temperature of a non-paper-passing portion) occurs in which a temperature of an end region where the recording material does not pass rises. In particular, the phenomenon frequently occurs in the image fixing apparatus with a film fixing method, in which a heat capacity of a heating-rotatable member is reduced for purpose of saving energy.
As one of measures against the rise of the temperature of the non-paper-passing portion, it is known that a cooling fan for cooling the non-paper-passing portion is provided. Japanese Patent Application Laid-Open No. 2008-058378 discusses that shutters moving according to a size of the recording material and an element for detecting the temperature of the non-paper-passing region portion of the fixing member are provided so as to adjust air volume by a cooling fan according to the detected temperature. Since the two shutters are moved by one motor, the apparatus can be simplified.
However, in the image fixing apparatus equipped with a cooling system for supplying air to the non-paper-passing portion in the image fixing apparatus by providing the cooling fan, when the above-described simplified apparatus is applied, the problems may occur as below.
FIG. 1A illustrates a schematic configuration of an image fixing device with a film fixing method including cooling fans and distribution of the temperatures of a ceramic heater. FIG. 1A illustrates a recording paper P, a heating portion 301 of the ceramic heater, and cooling fans 222 and 232. The recording paper P is conveyed with reference to a broken line B (conveyance reference) passing at a center of the ceramic heater in a longitudinal direction.
An amount of heat generation by the ceramic heater is adjusted based on a result acquired by a temperature detection element Th1 (illustrated in FIG. 4) provided at a point Q of a center portion of the ceramic heater. The point Q of the center portion of the ceramic heater is controlled to maintain a desired temperature (a target temperature). A solid line K indicates the distribution of the temperature of the ceramic heater when the amount of the heat generation at end portions C and D of the ceramic heater varies or when the recording paper P to be passed passes as being set closer to the end portion D side. The broken line J indicates that the recording paper P continuously passes while the state indicated by the solid line K is maintained. As illustrated in FIG. 1A, the variation in the amount of the heat generation at the end portions C and D of a heating portion 301 of the ceramic heater, and an amount of shifting from the point Q of the center portion of a path where the recording paper P passes cause a difference in the distribution of the temperature at the end portions C and D. The greater the amount of the variation and the amount of the shifting become, the larger the difference between the temperatures at the non-paper-passing portions G and H becomes. At this point, when the temperature of either one of the end portions C and D exceeds a threshold value T, a shutter is moved by a driving unit that can be used for both right and left shutters. At the same time, the cooling fan for cooling the one end portion starts cooling.
However, when the temperature of the other end portion does not need to be cooled at this point, even if the cooling fan is stopped, the shutter is opened with reference to the raising temperature of the one end portion. Consequently, heated air around the fixing device proceeds from an opening portion to the cooling fan out of operation via a duct, to raise the temperature of components included in the cooling fan. Thus, the cooling fan may be damaged or characteristics of the cooling fan may be significantly deteriorated. Further, since the duct for supplying air is reduced in length due to the reduced size of the device, the device is readily further impacted by the heated air around the image fixing device. To avoid the above-described problem, the driving unit for moving the shutters may be divided in two. However, that may complicate the device and raise costs.
Further, a detection unit may be provided for detecting an ambient temperature of the cooling fan. However, that also raises the costs.